1. Field of the Invention
The invention relates generally to reverse osmosis and ultrafiltration fluid separation processes, and is particularly applicable to water desalination and purification by reverse osmosis.
2. Prior Art
In reverse osmosis a feed fluid, eg. saline water, is pumped at an elevated working pressure into a pressure vessel containing semi-permeable membranes. Purified product water, eg. the permeate fluid fraction, permeates across the membranes and a more concentrated concentrate fluid fraction is rejected by the membranes at the working pressure. Pressure energy in the rejected concentrate fluid fraction can be recovered to assist in pressurizing the feed fluid during a pumping stroke.
Other inventors have recognized the importance of improving the efficiency of the reverse osmosis process by recovering energy of the concentrate fluid. Such devices are disclosed in U.S. Pat. Nos. 3,558,242 (inventor W. D. Jenkyn-Thomas) and 4,124,228 (inventor L. P. S. Wilson). It would appear that these patents have some practical difficulties relating to valve timing and these difficulties have been reduced considerably by the provision of a dwell means referred to in the present inventor's patent and copending application.
In the prior art, means to reduce the detrimental effects of concentration polarization occurring on the working face of the membranes usually include accumulators to maintain an essentially steady fluid flow across the membrane face to remove the excessively concentrated fluid adjacent thereto. To reduce concentration polarization multi-cylinder reciprocating reverse osmosis pumps have been contemplated and difficulties can arise due to the complexity of valve timing associated with the valves controlling flow of fluid relative to an expansion chamber adapted to receive pressurized concentrate fluid from the membranes to assist in pressurizing feed fluid in an associated pumping chamber. German Offenlegungsschrift No. 24 44 740 (inventor Tuchenhagen) discloses a three cylinder reverse osmosis pump using a rotary valve for directing fluid flow to and from the expansion chamber, the valve being driven directly from a crankshaft actuating pistons of the cylinders.